FIELD OF THE INVENTION
The invention relates to a device for contact-free sheet guidance in a sheet-fed printing press and, more particularly, to such a sheet guidance device having a sheet guiding surface, wherein there is provided at least one blast air nozzle having a cross section formed as a ring slot sector, and having an air guiding surface directed obliquely downwardly from the sheet guiding surface.
A sheet guidance device of this general type has become known heretofore from German Patent 19 07 083 wherein, in a guide baffle, blast air nozzles having a cross section in the form of a sector of a ring slot are provided in a surface of the guide baffle, and an air guiding surface is formed extending obliquely downwardly relative to the surface of the guide baffle, and defining the ring slot sector which is located opposite to the air guiding surface. The blast air nozzles of the conventional device shown in the German patent are provided with a guide baffle formed with punched or stamped-out tonguelike notches for producing a directionally focused airflow, with a shallow angle directed in the sheet-feeding direction towards the sheet to be transported which floats above the surface of the guide baffle. A geometrical bottleneck for the air flowing through the respective blast air nozzle is located at an air outlet location. This choke point is followed by a diverging guide surface acting as a diffuser and, in this region, a negative pressure typical for a diffuser is generated which sucks the sheet against the guide face formed by the tonguelike notches. Due to this suction, contact-free sheet guidance is no longer achieved. A consequence thereof is the occurrence of scratches, and smearing of the side of the sheet communicating with the guidance device. Comparable with the foregoing conventional device is the action of a device heretofore known from the published German Patent Document DE 41 13 465 A1.